1. Field of the Invention
The present invention relates to a method of and an apparatus for producing an electrode of a fuel cell. The output of the fuel cell with the electrode is high even when current density is high.
2. Description of the Related Art
In general, a fuel cell comprises a stack including a predetermined number of cell units which are electrically connected in series to one another. A collector is electrically connected to each of the cell units which are disposed at both ends of the stack. Further, an end plate is arranged outside of each of the collectors with an insulating plate interposed between the end plate and the collector to avoid electric leakage. Additionally, a backup plate may be arranged outside of each of the end plates in some cases. The stack, the insulating plates, and the collectors are interposed between the end plates or between the backup plates connected to each other by tie rods or the like.
The cell unit has an electrolyte electrode assembly 1 as shown in FIG. 8. The electrolyte electrode assembly 1 comprises an anode 2 and a cathode 3 which are connected to end surfaces of an electrolyte layer 4 respectively.
In this arrangement, the anode 2 comprises a water-repellent layer 6a and an electrode catalyst layer 7a which are stacked in order of mention on an electrode base material 5a. In general, the electrode base material 5a is made of carbon paper, carbon cloth, or the like. The water-repellent layer 6a is made of carbon black and polytetrafluoroethylene (PTFE). The electrode catalyst layer 7a is made of carbon black with Pt supported thereon. On the other hand, the cathode 3 is constructed in the same manner as the anode 2. Therefore, the constitutive elements of cathode 3 are denoted by reference symbols of the elements of the anode 2 but replacing xe2x80x9caxe2x80x9d with xe2x80x9cbxe2x80x9d, and detailed explanation thereof is omitted.
The electrolyte layer 4 comprises a membrane 8 impregnated with a liquid electrolyte. The membrane 8 may be a polymer membrane (see U.S. Pat. No. 5,525,436) made of basic polymer such as polybenzimidazole or porous SiC. On the other hand, the liquid electrolyte may be liquids capable of electrically conducting hydrogen ion such as phosphoric acid, sulfuric acid, and methanesulfonic acid.
The cell unit comprises the electrolyte electrode assembly 1 which is interposed between a pair of separators (not shown).
When the fuel cell is operated, a fuel gas such as a hydrogen-containing gas is supplied to the anode 2 via a gas flow passage in the separator, and an oxygen-containing gas such as air is supplied to the cathode 3. During this process, hydrogen in the fuel gas is ionized in the electrode catalyst layer 7a of the anode 2, and thus hydrogen ions and electrons are produced.
The produced hydrogen ions are moved in the cell unit, and arrive at the electrode catalyst layer 7b of the cathode 3. During this process, the electrons are extracted by an external circuit which is electrically connected to the collectors. The electrons are utilized as DC electric energy to energize the external circuit. After that, the electrons arrive at the cathode 3. The hydrogen ions and the electrons reaching the cathode 3 react in the electrode catalyst layer 7b with the oxygen in the oxygen-containing gas supplied to the cathode 3. Thus, H2O is produced. H2O is repelled by the water-repellent layers 6a, 6b of both electrodes 2, 3, and is promptly discharged. Accordingly, it is possible to prevent the liquid electrolyte in the electrolyte layer 4 from flowing outside together with H2O.
The anode 2 is produced, for example, by using a screen printing apparatus 10 shown in FIG. 9. At first, the electrode base material 5a (carbon paper or carbon cloth) is placed on a vacuum suction plate 12 through which a plurality of suction holes 11 each having a diameter of about 1 to 2 mm are provided at intervals of about 10 mm.
A suction jig 14, which is provided with a recess 13, is arranged under the vacuum suction plate 12. A tube section 15 protrudes on one side of the suction jig 14. An unillustrated suction mechanism is connected to the tube section 15. The atmospheric air around the vacuum suction plate 12 is sucked through the recess 13 by the suction mechanism. Accordingly, the vacuum suction plate 12 is positioned and fixed on the suction jig 14. Similarly, the atmospheric air around the electrode base material 5a is sucked through the suction holes 11. Accordingly, the electrode base material 5a is positioned and fixed on the vacuum suction plate 12.
A paste P1 for the water-repellent layer (see FIG. 10), which is prepared by dispersing carbon black particles and PTFE particles in a solvent such as ethylene glycol together with a surfactant, is applied onto the electrode base material 5a in this state.
Specifically, the paste P1 is supplied onto a screen 17 of a screen section 16 of a screen printing apparatus 10. Subsequently, as shown in FIG. 10, a squeegee 18 is displaced from the right end to the left end. Accordingly, the screen 17 is expanded from a frame member 19 toward the electrode base material 5a, and the paste P1 is applied onto the electrode base material 5a through the screen 17.
The paste P1 is coated on the electrode base material 5a by a predetermined thickness. The paste P1 is heated to remove the solvent by volatilization. Accordingly, the water-repellent layer 6a (see FIG. 8) is formed.
Subsequently, the electrode base material 5a, on which the water-repellent layer 6a has been formed, is positioned and fixed again on the vacuum suction plate 12 (see FIG. 9) in the same manner as described above, and a paste P2 for the electrode catalyst layer is supplied onto the screen 17. The paste P2 is prepared by dispersing carbon black particles with Pt supported thereon in a solvent such as ethylene glycol. The squeegee 18 of the screen printing apparatus 10 is displaced from the right end to the left end in the same manner as described above. Accordingly, the paste P2 is coated onto the water-repellent layer 6a through the screen 17.
Finally, the electrode base material 5a, the water-repellent layer 6a, and the paste P2 are pressed and heated. During this process, the solvent in the paste P2 is removed by vaporization. Accordingly, the electrode catalyst layer 7a (see FIG. 8) is formed. As a result, the anode 2 is completed. The cathode 3 is produced in the same manner as the anode 2.
If the water-repellent layers 6a, 6b are unnecessary, the steps of applying and drying the paste P1 are omitted. That is, the electrode catalyst layers 7a, 7b are directly formed on the electrode base materials 5a, 5b by applying the paste P2 onto the electrode base materials 5a, 5b, and then drying the paste P2.
The carbon paper or the carbon cloth used for the electrode base material 5a, 5b is a porous member having a porosity of 70 to 90%. For this reason, when the paste P1 is applied after positioning and fixing the electrode base material 5a, 5b by suction as described above, the paste P1 enters pores 20 of the electrode base material 5a, 5b as shown in FIG. 11. Therefore, a large number of depressions 21 are formed on the water-repellent layer 6a, 6b. If the paste P2 is applied and dried in this state, a large number of depressions 21 are also formed on the electrode catalyst layer 7a, 7b. 
The electric charge distribution is not uniform in the electrode catalyst layer 7a, 7b having the large number of depressions 21. The conductivity of the electrodes 2, 3 of the fuel cell is lowered, and the internal resistance of the fuel cell is increased. Therefore, the output of the fuel cell is lowered, especially when current density is high.
In order to solve the problem as described above, it is considered to be effective that the electrode base materials 5a, 5b are mechanically positioned and fixed by using a fixing jig such as a clamp. However, in this procedure, a part of the electrode base materials 5a, 5b is covered with the fixing jig. Therefore, the paste P1 or the paste P2 is not applied to the part. As a result, the surface area of the electrode catalyst layers 7a, 7b in which electrochemical oxidation-reduction reaction occurs, in other words, the electrode area is decreased. Therefore, the output density based on the volume of the fuel cell is consequently lowered.
In order to avoid the decrease in output density of the fuel cell, the portion, to which the paste P1 or the paste P2 is not applied, may be trimmed. However, the number of steps is increased thereby. Therefore, it is impossible to efficiently manufacture the electrode of the fuel cell.
In any case, it is necessary to position and fix the electrode base materials 5a, 5b with the fixing jig and detach the electrode base materials 5a, 5b from the fixing jig. These operations are quite complicated and require a long period of time.
A principal object of the present invention is to provide a method of and an apparatus for producing an electrode of a fuel cell in which a substantially flat electrode catalyst layer can be formed, and thus it is possible to construct a fuel cell having a high output even at a high current density.
According to the present invention, there is provided a method of producing an electrode of a fuel cell, comprising the steps of:
holding an electrode base material by electrostatic attraction;
applying a first paste to the electrode base material; and
drying the first paste to form a water-repellent layer.
In this production method, the paste for the water-repellent layer is applied while the electrode base material is held by electrostatic attraction. Therefore, during this process, the paste for the water-repellent layer is not sucked, and it does not enter pores of the electrode base material. Accordingly, it is possible to form the water-repellent layer which includes an extremely small number of depressions.
Further, a substantially flat electrode catalyst layer can be formed on the water-repellent layer. Accordingly, the electric charge distribution is substantially uniform in the electrode catalyst layer. Therefore, it is possible to obtain an electrode of the fuel cell having a high conductivity.
The fuel cell having the electrode produced as described above has a low internal resistance. Therefore, it is possible to construct the fuel cell which has a high output even when current density is high.
It is preferable that the electrode catalyst layer is formed in the same manner as the water-repellent layer. In other words, it is preferable that the method further comprises, in addition to the steps described above, the steps of:
holding the electrode base material on which the water-repellent layer is formed by the electrostatic attraction;
applying a second paste to the water-repellent layer; and
drying the second paste to form an electrode catalyst layer.
When the electrode base material is held by electrostatic attraction, it is possible to form the electrode catalyst layer which includes an extremely smaller number of depressions. Therefore, it is possible to obtain an electrode of the fuel cell having a more satisfactory conductivity. Consequently, it is possible to construct the fuel cell having a higher output.
When the water-repellent layer is not provided, the electrode catalyst layer may be directly formed on the electrode base material. That is, according to another aspect of the present invention, there is provided a method of producing an electrode of a fuel cell, comprising the steps of:
holding an electrode base material by electrostatic attraction;
applying a paste to the electrode base material; and
drying the paste to form an electrode catalyst layer.
Also in this case, it is possible to obtain a substantially flat electrode catalyst layer which includes a small number of depressions. Consequently, it is possible to construct the fuel cell having a higher output.
It is noted that the electrode material is usually a porous member having a porosity of 70 to 90%. Therefore, if the voltage applied to the electrodes for the electrostatic attraction is low, the electrode base material is not sufficiently held in some cases. On the other hand, if an extremely high voltage is applied, it is sometimes difficult to take out the electrode base material, because electrostatic attraction force remains even after the power source is shut off. In order to reliably avoid the inconvenience as described above, it is preferable that a material having a porosity of 70 to 90% is used as the electrode base material, and a voltage of 1000 to 6500 V is applied to the electrodes for the electrostatic attraction.
According to still another aspect of the present invention, there is provided an apparatus for producing an electrode of a fuel cell, comprising:
a mechanism for effecting electrostatic attraction; and
a mechanism for applying to an electrode base material at least one of a first paste to form a water-repellent layer in the electrode of the fuel cell and a second paste to form an electrode catalyst layer in the electrode of the fuel cell,
wherein the electrostatic attraction mechanism holds the electrode base material by electrostatic attraction and the paste-applying mechanism applies the paste for the water-repellent layer or the paste for the electrode catalyst layer to the electrode base material.
When the apparatus is constructed as described above, it is possible to apply the paste for the water-repellent layer while the electrode base material is held by electrostatic attraction. Therefore, it is possible to prevent the paste for the water-repellent layer from being suctioned during this process so that the paste for the water-repellent layer does not enter pores of the electrode base material. Therefore, the water-repellent layer including an extremely small number of depressions is formed. Accordingly, the electrode catalyst layer formed on the water-repellent layer is substantially flat as well. The electric charge distribution is substantially uniform in the electrode catalyst layer which is flat as described above. Therefore, it is possible to obtain the fuel cell having a low internal resistance. Consequently, it is possible to construct the fuel cell which has a high output even when the current density is high.
It is preferable that the apparatus further comprises a suction mechanism for holding by suction an electrostatic attraction plate of the electrostatic attraction mechanism. The electrostatic attraction plate holds the electrode base material. Accordingly, it is possible to position and fix (hold) the electrostatic attraction plate easily and reliably.
Preferred examples of the apparatus for producing the electrode of the fuel cell as described above may include a screen printing apparatus.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.